This section provides background information related to the present disclosure which is not necessarily prior art.
In many four-wheel drive vehicles, a transfer case is installed in the drivetrain between the primary (i.e. rear) driveline and the secondary (i.e. front) driveline and is operable for selectively and/or automatically transmitting drive torque generated by the powertrain to all four wheels so as to provide a four-wheel drive (4WD) mode of operation in addition to a two-wheel drive (2WD) mode of operation. In some transfer cases, a mode shift system permits the vehicle operator to selectively shift between the 2WD mode wherein all drive torque is transmitted to the primary wheels via the primary driveline and a “part-time” or locked 4WD mode wherein a dog clutch rigidly interconnects the secondary driveline to the primary driveline. As an alternative, the positive-locking dog clutch may be replaced with a torque transfer coupling that is configure to provide automatic or adaptive control over the amount of drive torque transferred to the secondary driveline so as to define an “on-demand” 4WD mode. Typically, the torque transfer coupling includes a multi-plate friction clutch and a power-operated clutch actuator that is controlled by a traction control system in response to certain vehicle operating conditions (i.e. vehicle speed, acceleration rate, steering angle, braking status, etc.) for varying a clutch engagement force applied to the friction clutch which, in turn, varies the drive torque distribution between the primary and secondary drivelines.
In a large majority of modern transfer cases, the dog clutch or the friction clutch, hereinafter cumulatively referred to as the “mode” clutch, is operably arranged to surround a primary axis defined by the mainshaft which interconnects an output of the powertrain to an input of the primary driveline. The front output shaft is rotatable about a secondary axis that is offset from the primary axis. A transfer assembly, such as a chain drive assembly, is connected to the front output shaft and the mode clutch is operable to selectively/automatically couple the transfer assembly to the mainshaft so as to transfer drive torque to the front output shaft. Typically, a sump of lubricant is provide in proximity to the secondary axis and means are also provided for pumping the lubricant from the sump to the mainshaft so as to lubricate the bearings and other rotary components associated with the transfer assembly and the mode clutch.
In some transfer cases, a shaft-driven lube pump such as, a gear pump or gerotor pump, is associated with the mainshaft for pumping lubricant from the sump in response to rotation of the mainshaft. In some other transfer cases, a pumpless lubrication system is employed to convey the lubricant upwardly from the sump toward the mainshaft without the use of a lube pump. For example, U.S. Pat. No. 8,650,980 discloses a transfer case equipped with a pumpless lubrication system which utilizes the rotation of the gear-type transfer assembly to provide the fluid pumping action.
It is also known to disconnect the secondary driveline when the motor vehicle is operating in its 2WD mode to prevent the secondary wheels from “back-driving” the secondary driveline in an effort to reduce parasitic drag and improve fuel economy. Most commonly, a disconnect coupling is installed in the secondary axle assembly between the secondary wheels and the secondary differential unit. The disconnect system can include a pair of hub disconnect couplings associated with the secondary wheel or a shaft disconnect coupling disposed between two shaft segments of an axleshaft. Examples of dog-type and multi-plate disconnect couplings are shown in U.S. Publication No. US2009/0321208. In four-wheel drive vehicles equipped with such secondary driveline disconnect systems, the front output shaft and the transfer assembly within the transfer case are not driven during operation in the 2WD mode.
However, a need exists to still provide means for pumping lubricant from the sump to the mainshaft for lubrication of the mode clutch as well as the bearings supporting other rotary components on the mainshaft when the mode clutch is released and the vehicle is operating in its 2WD mode. Since lubricant is not conveyed by the transfer assembly, such transfer cases typically include the previously noted shaft-driven gerotor lube pump or, in the alternative, an electrically-driven lube pump for providing on-demand lubrication. As an alternative, a pumpless transfer case lubrication system is disclosed in U.S. Pat. No. 8,776,950 which is configured to periodically actuate the on-demand mode clutch when the secondary axle assembly is disconnected so as to drive the transfer assembly for use as the lubricant conveying device.
In view of the above, it is recognized that a need exists to continue development of pumpless transfer cases for use in four-wheel drive vehicles equipped with a disconnectable secondary driveline.